1. Field of the Invention
The present invention relates generally to light emitting diode (LED) driver circuitry and in particular to LED driver circuitry that utilizes brightness control having non-linear compensation circuitry.
2. Description of Related Art
Light emitting diodes have many applications, including backlighting for liquid crystal displays (LCDs). A typical backlight system may introduce non-linearities between a requested brightness and the actual light output. FIG. 1 is a graph from a typical LED data sheet depicting the forward current of the LED versus the relative luminous flux. As can be seen, the luminous output does not increase linearly with the forward current. By way of example, for a forward current value of 20 mA, the relative luminous flux value is 1 but when the current is increase by a factor of four (80 mA), the flux value is only at 3.5.
In addition to non-linearities introduced by the LEDs themselves, there are issues relating to non-linearities of human perception. FIG. 2 includes a set of curves taken from “Perceptual Uniformity of Digital Imaging” by Charles Poynton (2010), which illustrate the non-linear human response to various levels of light. Among the curves is a curve labeled 0.01 L*(T) which is based on the CIE L* model of human response to lightness produced by the International Commission on Illumination. As can be seen from these curves, the human perception of light increases dramatically for changes at lower light levels and becomes relatively linear at higher levels.
For many LED drive applications, tight control of LED linearity is important. By way of example, in Content Aware Backlight Control (CABC) applications (sometimes also referred to as Content Adaptive Brightness Control), the LED backlighting brightness is intelligently controlled by analyzing the LCD content. Although this technique reduces power consumption, the LCD pixel brightness must be precisely matched by the changes in the LED backlight brightness. Non-linearities in the backlight system can make this matching more difficult.
In some applications, it is desirable to control the backlighting in a manner such that the LEDs are ramped up and dimmed in a non-linear manner. By way of example, a pleasing diming action can be produced by turning the back lighting off starting at a fast rate and ending at a much slower rate. Other logarithmic or exponential functions for backlighting control are sometimes desired.
Prior art approaches to compensating for non-linearities and for creating non-linearities frequently use a look-up table of one form or another. The problem with this approach is that a large number of look-up table points must be used to achieve a reasonable degree of compensation accuracy thereby making programming relatively difficult and requiring an increased amount of memory. Hardcoded look up tables are more space efficient but provide less application flexibility. Analog solutions generally do not provide programmability to support multiple curve shapes.
There is a need for improved LED drive circuitry having non-linear compensation that can provide precise compensation, can be easily re-programmed and which enables multiple compensation curves to be easily implemented. As will become apparent to those skilled in the art upon a reading of the following Detailed Description of the Invention together with the drawings, the present invention successfully addresses these short comings of the prior art.